The present invention pertains to an improved cutting device for trimming adjacent edges of pieces of fabric being joined by means of flat seam stitching with a covering stitch in a sewing machine. When two pieces of material are to be joined by seaming in a manner whereby their sewn edges are disposed in co-planar and contiguous relation, it is common practice to feed the two pieces into the machine so that the finished and joined edges are formed adjacent to the original edges of each piece which are severed from the latter immediately prior to the seaming operation. This is accomplished by positioning the pieces so that the major portion of both are disposed on the same horizontal plane and with the adjacent edges of each being turned at right angles to said horizontal plane and in abutting relationship with one another. These abutting edges are caused to enter a guide formed in the machine's presser foot and are severed from the two pieces just prior to the latter being subjected to the seaming operation of the flat-seam and covering stitch type.
To perform seaming operations of this type which are commonly referred to as cut-stitch operations, trimming devices are provided which include a fixed blade disposed horizontally in the base of the presser foot and which extends perpendicular to the direction of advance of a workpiece. Such trimming devices also include a movable blade carried by a movable support which locates it in operative association with the fixed blade and the movable support is operatively connected to any suitable actuating means forming a part of the machine itself. The movable support is also provided with a biasing means which serves to maintain contact between the fixed and movable blades. The presser foot is attached to the lower end of the fabric presser bar which is supported in the frame of the sewing machine and in a known manner is continuously biased by spring means associated with said presser bar in the direction of the usual needle plate which is mounted in the machine's worksurface that supports the workpieces during the sewing operation. A conventional transport means or feed dogs cooperate in a known manner with the underside of the presser foot to effect advance of the workpiece during the seaming operation. The biasing means operatively associated with the movable support is adapted to act on the movable blade so as to continually urge it into contact with the fixed blade even when the presser foot is being displaced in an upwardly direction away from the needle plate.
In the known type of trimming devices described above, the fixed blade is supported in the base portion of the presser foot so as to be in relatively close proximity with the needle plate and the movable blade is located above and in operative association with said fixed blade. The biasing means for the movable support serves to maintain the two blades in operative contact by pressing downwardly on the movable blade thus effecting its actuation in a positive manner. This downward force on the movable blade coupled with the biasing means acting on the presser bar subject the feed dogs, to what is considered, an excessive amount of downward force by the presser foot. As a result of this the feed dogs in order to perform their intended function must overcome the combined forces of both biasing means associated with the presser bar and movable blade.
It must be understood that both of these biasing means must be capable of producing selected and accurate amounts of force which differ one from the other in view of the function each is caused to perform. In particular, the biasing means for the presser bar should be such that the force it exerts while cooperating with the feed dogs will not have any damaging effects on the structure of the workpiece being advanced therebetween. Additionally, the biasing means for the movable support must be sufficient to maintain operative contact between the two blades without subjecting them to too great a biasing force which would result in excessive wear and a shortened life expectancy of said blades. These factors are considered serious in known trimming devices for it is quite difficult and time consuming to obtain the most desirable setting of each of the two biasing means so that the forces they provide will be the most advantageous for the particular characteristics of the material forming the workpiece.
Additionally, the biasing means for the movable supports in the known trimming devices are subjected to compression forces caused by the upward movement of the feed dogs which passes through the fabric to the presser foot and in a direction that opposes the spring force pressing on said fabric and being directed in an upwardly direction, said compression forces are transmitted to the biasing means acting on the movable blade. This condition occurs when contact between the two blades is such that their cutting edges are in spaced relation which defines their inactive or rest position and places undue stress on the movable blade. This condition also creates an unbalanced condition in the presser foot which can be attributed to the biasing means of the movable support that creates a greater degree of compression on the side of said presser foot at which said movable blade is operating.
As a result of the increased forces on one side of the presser foot, the fabric being advanced by the latter has a tendency to become misaligned or in other words, the fabric is urged to rotate out of alignment with the intended path it should follow during its advancement.
It is a general object of the present invention to eliminate the described disadvantages prevalent in known trimming devices.
A further object is that of lightening the loads on the elements which are in frictional contact and in contact with the fabric so as to substantially reduce wear between the blades and provide a means to maintain desired control of the forces acting on the presser foot during advancement of the fabric so that the latter accurately follows its intended and desired path toward the sewing zone.